target/riscv: add fault-only-first unit stride load

target/riscv: add fault-only-first unit stride load

The unit-stride fault-only-fault load instructions are used to
vectorize loops with data-dependent exit conditions(while loops).
These instructions execute as a regular load except that they
will only take a trap on element 0.

Signed-off-by: LIU Zhiwei <zhiwei_liu@c-sky.com>
Reviewed-by: Alistair Francis <alistair.francis@wdc.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Message-Id: <20200701152549.1218-9-zhiwei_liu@c-sky.com>
Signed-off-by: Alistair Francis <alistair.francis@wdc.com>